1. Field of the Invention
The present invention relates to a silver halide tabular grain emulsion useful in light-sensitive photographic materials. More in particular, the present invention relates to a silver halide tabular grain emulsion having a specific grain morphology to be used in radiographic materials.
2. Background of the Invention
Silver halide tabular grains, their preparation and use in photographic emulsions, are widely known. Silver halide tabular grains are crystals possessing two major faces that are substantially parallel. They have been extensively studied in the literature since photographic emulsions containing these grains appeared to offer some significant advantages over photographic emulsions containing round or globular or cubic grains. Tabular grains usually have polygonal (i.e., triangular or hexagonal) parallel crystal faces, each major face of which is usually greater than any other crystal face of the grain. These tabular grains are conventionally defined by their aspect ratio (namely AR) which is the ratio of the diameter of the grain to the thickness. Tabular grains offer significant technical and commercial advantages apparent to those skilled in the art. The most important advantages of tabular grains can be summarized as follows:                1. Tabular grains have a high surface to volume ratio so that a large amount of sensitizing dye can be adsorbed on the surface, and a high development rate and covering power can be obtained.        2. Tabular grains tend to lie parallel to the surface of the support base when emulsions containing them are coated and dried so that it is possible to reduce the thickness of the coated layer and accordingly to increase sharpness.        3. When a sensitizing dye is added to tabular grains, the extinction coefficient of the dye is greater than the extinction coefficient for the indirect transition of the silver halide so that in X-ray materials it is possible to obtain a relevant reduction in cross-over, thereby preventing any worsening of quality.        4. Tabular grains are usually very thin and so the amount of radiation absorbed per grain (proportional to the thickness) is low and there is low fogging resulting from natural radiation on aging.        5. Tabular grains show low light scattering and the images obtained from them have a high resolution.        
In spite of all these advantages, tabular grain emulsions tend toward more dispersed grain populations than can be achieved in the preparation of silver halide conventional grains, e.g., cubic or octahedral grains. This has been a concern since reducing grain dispersion within an emulsion is generally recognized to increase the imaging consistency of the emulsion. Grain dispersion concern relates to (1) the presence of non-conforming grain shapes, such as, for example, octahedral, cubic, or rod shapes, (2) the variance of the grain size distribution, and (3) the variance of the grain thickness distribution.
Non-conforming grains can interact differently with light and exhibit some undesirable properties. For example, faces of non-tabular grains are randomly oriented with respect to the support base, octahedral grains exhibit lower covering power and greater thickness, and rod grains can self develop in the absence of light, thereby increasing fog.
On the other hand, even a population of grains having a common shape can have a high variability or high dispersion in terms of grain size and thickness distribution. A common method for quantifying grain size distribution is to extract a sample of individual grains, calculate the corresponding diameter for each grain (D1→n, wherein n is the number of extracted grains), calculate the average diameter (Dm=Σ1→nD/n), calculate the standard deviation of the grain population diameters (Sd), divide the standard deviation (Sd) by the average diameter (Dm) and multiply by 100, thereby obtaining the coefficient of diameter variation (COVd) of the grain population as a percentage. A similar method is used for quantifying the grain thickness distribution and obtaining the coefficient of thickness variation (COVt).
Accordingly, several solutions have been proposed in the art to reduce the COVd of tabular grain emulsions. Monodispersed tabular grain emulsions and methods to prepare them are disclosed in several patents and patent applications attempting to obtain tabular grains having reduced COVd by controlling various parameters during nucleation and ripening of the silver halide emulsion. The most important nucleation conditions to be kept under control for obtaining monodispersed tabular grain emulsions are temperature, gelatin concentration, addition rates of silver salt solution, addition rates of alkali halide solution, stirring rate, iodide content in the alkali halide solution, amount of silver halide solvent, pH of the dispersing medium, concentration of bromide ions in the reaction vessel, molecular weight of dispersing medium, iodide content in the vessel at the start, and the like. Similarly, the most important ripening conditions are temperature, dispersing medium concentration, silver halide solvent concentration, pBr, and addition rates of silver salt solution.
Saito in U.S. Pat. No. 4,301,241 describes a process for forming a silver halide emulsion containing multiple twin crystal grains and a narrow grain size distribution. The examples report multiple twin crystal grain silver bromoiodide emulsions having an average grain size from 0.86 to 1.023 μm and a COVd of from 11.6% to 13.6%.
Saitou et al. in U.S. Pat. No. 4,797,354 disclose a silver halide emulsion comprising hexagonal tabular grains with an “adjacent edge ratio” of from 2/1 to 1/1 accounting for 70% to 100% of the projected area of all the grains, and further that said hexagonal tabular grains are monodisperse and have an average aspect ratio from 2.5:1 to 20:1. The term “adjacent edge ratio” is referred to as the ratio of the longest edge length to the shortest edge length of each hexagonal tabular grain. Accordingly, the definition of “adjacent edge ratio” is a measure of the hexagon regularity.
U.S. Pat. No. 4,722,886 describes a process to form a monodispersed silver halide tabular grain emulsion comprising the steps of adding silver nitrate to a reaction vessel with a reaction solution comprising a bromide ion concentration of from 0.08 to 0.25 N to form silver halide nuclei, adding a basic silver halide solvent (e.g., ammonia solution) to achieve 0.02 to 0.2N of such a silver halide solvent after at least 2% by weight of the total silver has been added to the vessel, stopping silver nitrate addition for a time period of from 0.5 to 60 minutes at a bromide ion concentration of from 0.005 to 0.05 N, neutralizing at least part of the present solvent, and growing the formed silver halide grains by adding silver and halide (Br or Brl) soluble salts by balanced double jet.
U.S. Pat. No. 4,798,775 discloses a process to obtain monodispersed tabular grains comprising the steps of forming silver halide nuclei with a silver iodide content of from 0 to 5% in the mother liquor, by maintaining the pBr in the reaction vessel between 2.0 and −0.7 for at least the initial half of the nucleation time, ripening the nuclei formed in the nucleation step by maintaining the concentration of silver halide solvent from 10−4 to 5 moles per liter of mother liquor, and growing the seed grains by addition of silver and halide soluble salts or by addition of silver halide fine grains.
U.S. Pat. No. 4,801,522 discloses a process to form silver halide tabular grains having a thickness of from 0.05 to 0.5 μm, average grain volume of from 0.05 to 1.0 mm3 and a mean aspect ratio higher than 2:1 comprising the steps of adding silver nitrate to a reaction vessel comprising a bromide ion concentration of from 0.08 to 0.25 N (pBr=1,1-0,6), adding ammonia solution to achieve 0.002 to 0.2N after at least 2% of the total silver has been added to the vessel, and adding silver and halide (Br or Brl) salts by balanced double jet.
U.S. Pat. No. 5,013,641 describes a process of forming monodispersed silver halide emulsions comprising (a) combining silver nitrate and sodium bromide in gelatin solution, (b) adding NaOH to adjust the pH to greater than 9, (c) allowing digestion of the nucleated particles, (d) adjusting the pH to below 7 by acid addition, and (e) adding silver nitrate and sodium halide to grow the nucleated particles.
U.S. Pat. No. 5,254,453 discloses a process for forming monodispersed silver bromide or bromoiodide grains with COVd lower than 25%, thickness of from 0.05 to 0.5 μm, mean aspect ratio higher than 2, and diameter of from 0.2 to 3 μm comprising the following steps: (a) digesting the nucleated particles in a basic silver halide solvent at a concentration of from 0.0015 to 0.015 N and (b) neutralizing said basic solvent after digestion and before growing.
EP 569,075 discloses a process of forming monodispersed silver bromide or bromoiodide tabular emulsions with average aspect ratio higher than 2, an average thickness of from 0.15 and 0.30 μm, and a COVd of from 0.15 to 0.45 wherein the process is characterized by (a) providing a gelatin/bromide solution at a pBr of from 1.0 to 2.0, (b) nucleating by consuming less than 10% of the total silver nitrate used, (c) making a first double jet growth (consuming at least 10% of the total silver nitrate used) at a pBr value of from 1.0 and 2.5, and (d) making a second double jet growth (consuming at least 40% of the total silver nitrate used) at a pBr value higher than 2.7.
EP 577,886 describes a process of forming monodispersed silver bromide or bromoiodide tabular emulsions with average aspect ratio of from 2 to 8, and a COVd lower than 30. The process comprises the following steps: (a) performing a nucleation step by balanced double jet by precipitating at most 5% of the total silver halide, (b) ripening the formed nuclei, (c) performing at least one growing step by balanced double jet at pBr lower than 2, (d) ultrafiltrating the reaction mixture during the precipitation steps with an ultrafiltration flux equal to or greater than the sum of the flow rates of the silver and halide ion solutions.
Grzeskowiak, in U.S. Pat. No. 5,028,521, discloses a process for preparing monodispersed silver halide tabular grain emulsions having an aspect ratio from 3:1 to 12:1 consisting in (a) preparing a bromide/gelatin mixture at pBr of from 0.7 to 1.0, (b) adding silver nitrate and further halide to maintain excess of bromide, (c) adding ammonia to achieve at least 0.05N after at least 20% by weight of the total silver is added, (d) adding further silver nitrate and halide by balanced double jet, by maintaining an ammonia concentration of at least 0.03N.
EP 588,338 describes a process characterized by specific nucleating conditions, that comprise (a) adding from 0.30 to 9.0% by weight of the total amount of soluble silver salt to a vessel containing 0.08 to 0.25 M aqueous soluble halide salt (b) adding a solution of ammoniacal base when 0.30 to 9% by weight of the total amount of soluble silver salt has been added, (c) adding soluble silver salt to growth the pBr from 1.3 to 2.3, and (d) adding soluble silver and halide salts to grow tabular grains.
A few solutions have been proposed in the art to reduce or control the COVt of tabular grain emulsions. The thickness of grains can be controlled by optimally selecting parameters affecting supersaturation at the time of nucleation, such as gelatin concentration, gelatin type, temperature, iodide concentration, pBr, pH, ion-supplying rate and stirring rate. Highly supersaturated conditions during nucleation generally favor the reduction of thickness.
EP 515,106 discloses a silver halide emulsion comprised of silver halide hexagonal tabular grains having even-numbered twin planes parallel to the major face and a maximum adjacent edge ratio of 2.0 to 1.0, a COVd within a range of from 21% to 29% and a COVt of 20% or less.
U.S. Pat. Nos. 5,275,929 and 5,302,499 disclose silver halide tabular grain emulsions of aspect ratio greater than 10 sensitized in the red region wherein the thickness of the silver halide grains is about 0.14 to 0.17 μm to minimize the spectral reflectance in the region of the spectrum where the emulsion has its maximum sensitivity. The specification is silent about coefficients of variation of diameter and/or thickness.
U.S. Pat. No. 5,906,914 and JP-A-7-191425 disclose tabular grain emulsions having a COVd lower than 20% and a limitation with respect to a ratio of a variation coefficient of twin plane spacing to a variation coefficient of grain thickness.